The present invention relates to playback instruments for record discs, particularly discs carrying high density recordings, and to discs for use with such instruments.
Information carrier discs, or record copies, carry information to be played back in the form of a spiral, modulated groove formed in a surface which is flat, at least when the record is new. Rigid record copy discs have a tendency to deviate from this ideal planar shape, for example, if they become warped due to stresses during storage. The prior art long-playing phonograph records which have a diameter of 30 cm, exhibit at their outer edges deviations from a flat plane of up to .+-.2 mm. Due to the relatively low rate at which such a record disc rotates during playback, this deviation has no noticeable adverse effect.
However, the playback of rigid discs recorded according to the dense storage technique, i.e. having a large number of groove turns per mm. and carrying high frequency information, requires a relatively high rate of rotation. Due to this high rate of rotation and the extremely small transverse dimensions of the groove being scanned only very slight deviations of the plane being scanned from a flat surface is permissible. For example, there exists the requirement that with a playback speed of 500 rpm and a disc diameter of 210 mm, a maximum deviation of 0.15 mm from a flat plane must not be exceeded. In order to meet such requirements, the disc must be aligned on the playback instrument to have a planar form. Centering of the disc must also be effected within very close tolerances. For example, the maximum acceptable eccentricity of the groove spiral relative to its center of rotation may be 0.02 mm.